Handle moved messages, part 2

[gnprice]

With #787 we implemented most of #150, handling moved messages. But as that PR description said:

Fixes most of https://github.com/zulip/zulip-flutter/issues/150. (After this lands we should do a quick scan to identify anything else that needs updating on message moves — there are several new data structures in the store that we've merged in recent weeks.)

Then we didn't do that scan.

Places that still need updating on message moves include:

• [ ] Unreads (which has a TODO comment for it)
• [ ] RecentSenders (that's one of those recently-merged data structures)
• … OK and I think that's it, having now scanned through what's in the store. (Another recently-merged data structure is RecentDmConversationsView, but that's only about DMs and those can't be moved.)